MPO Canopus - Time of Minimum Calculator
Variable star observers in particular use the time of minimum (usually the primary
minimum) as a reference point. By comparing the measured time of minimum for a given set
of data to the predicted time based on a known period and initial time of minimum, you can
determine the accuracy of the stated period and if the period might be getting longer or
shorter. Asteroid observations can use this concept to advantage to determine when a given
feature in the lightcurve might reoccur or when to work the asteroid to catch a missing
part of the curve.
Canopus uses the Hertzsprung method as detailed by Henden and Kaitchuck in their book
to find the time of minimum and generate an ephemeris that shows the times of minimum past
Use the period search routines in Canopus to find the period of the data before
beginning unless you already have an assumed period. In which case, you still need to plot
the observation data. The initial calculations to find the time of minimum are based on
days, regardless of the configuration settings. The TOM Calculator can work in either
hours or days once youve found the TOM.
After generating a period, you start the TOM calculator by first telling Canopus the
initial guess for time of minimum and the other search parameters. The initial guess is
automatically entered as the JD of the data point with the faintest magnitude. Once the
program finds the TOM, it displays the TOM Calculator.
Here you can generate an ephemeris for past and future times of minimum for planning
future observations or comparing the derived values against actual observations.
When working with the TOM calculator, you must use data from one and only
one session. If you include data from more than one session, the results are
unpredictable. If you have multiple sessions covering the same time of
minimum, e.g., data from different observers, you can then compute an improved average
value and error using a statistical calculator or spreadsheet.